A Multi-Pixel Photon Counter (MPPC: Registered Trademark) has been known as a photodiode array composed of a plurality of avalanche photodiodes (APDs) which are operated in the Geiger mode. The plurality of APDs are connected in parallel, and a reverse bias voltage higher than a breakdown voltage is applied to those, and those are operated in the Geiger mode. The MPPC includes quenching resistors which are connected in series to the avalanche photodiodes. An electric current from the avalanche photodiode to the quenching resistor, and the bias voltage to the avalanche photodiode drops down to the breakdown voltage. Thereafter, recharging causes the reverse bias voltage to return to the voltage in Geiger mode.
Because the MPPC has a temporal characteristic better than those of other detectors, it is considered promising that the MPPC is used as a detector such as a Time-Of-Flight (TOF) measurement system PET (Positron Emission Tomography) device. For photon measurement in a PET device, a higher temporal resolution is required. In order to improve a temporal resolution, it is necessary to acuminate a shape of an output waveform from the MPPC. The following non Patent Document 1 shows that an output pulse is acuminated, and the temporal resolution is improved by adding a capacitance (Cq) parallel to a quenching resistor.